In recent years, consumer electronic devices such as digital cameras, Personal Digital Assistants (PDAs), smart phones, and digital audio and video recorders have driven a strong market demand for removable data storage components. The electronics industry has responded to this demand with products known generically as “memory cards.” A memory card usually contains one or more semiconductor memory chips within an industry-standard housing with dimensions that allow it to be used in conjunction with different devices from various manufacturers. Memory cards typically also have connectors on an external surface that allow electrical connections to the circuitry of consumer electronic devices. Examples of types of memory cards include PC Cards, MultiMedia Cards, CompactFlash Cards, and Secure Digital Cards. These devices are made in accordance with standards promulgated by trade associations such as the Personal Computer Memory Card International Association (“PCMCIA”) and the MultiMedia Card Association (“MMCA”).
An exemplary memory card, namely, a MultiMedia Card (“MMC”) 10, is illustrated in top view, cross-sectional side view, and bottom view in FIGS. 1–3, respectively. The MMC illustrated has standardized dimensions of 32 mm long, 24 mm wide, 1.4 mm thick, and typically includes a memory capacity of between 2 and 256 megabytes (“MB”) of memory, which is accessed through seven contacts 11 located on the bottom surface of the MMC using, e.g., a standard serial port interface (“SPI”) interface. A simple chamfer 12 on one corner of the MMC prevents incorrect insertion of the MMC into a connector in a host device.
The exemplary prior art MMC shown in FIGS. 1–3 comprises a rectangular substrate 13, such as a printed circuit board (“PCB”), and one or more semiconductor memory dies 14 or “chips” mounted on and electrically connected thereto using, e.g., a layer of adhesive 15 and conventional wire bonds 16, respectively. Surface mounted passive components, e.g., resistors, may also be mounted on and connected to substrate 13. Contacts 11 are connected through substrate 13 to memory circuits defined by foregoing components and serve as input-output terminals of card 10.
When the components have been mounted on and connected to the substrate 13, prior art methods included a step in which chip 14 is protectively encapsulated by a “glob-topping” process. This step was necessary due to the high-pressure, high-temperature injection of thermoplastic material that would occur at a later stage. The high-pressure injection and high temperature can damage a microchip and other small electronic components, particularly wire bonds. In the glob-topping step a glob of a viscous encapsulant is dispensed onto the top surface of the chip and is allowed to flow over the chip's sides to the surface of the substrate. The encapsulant is cured to form a protective envelope 18 over the chip. An external cover or housing 19 (shown by the dotted outline in FIG. 1) of thin sheet metal or plastic is installed over the substrate 13 assembly by embedding the top surface of the assembly in a bed of adhesive contained in the housing 19.
Prior art methods for making memory cards are, to a large degree, concerned with properly positioning and fixing electronic components, modules or assemblies inside the memory card. This concern is due to the fact that if the electronic components are not properly affixed they will be moved to random positions during injection of the thermoplastic material into a card-forming cavity. This is a particular problem in the prior art processes because the injection occurs under the influence of rather high pressures. Prior methods for making memory cards included the use of relatively large, mechanical holding devices having hard, sharply defined bodies for holding the electronic components in place during injection of thermoplastic materials. The use of such holding devices can limit the positioning options for the electronic components in the memory card. The positioning limitation also may cut down on the size and number of electronic components that can be placed in such memory cards. This limitation in turn limits the amount of memory that can be put into an MMC.
Additionally, due to differences in the coefficients of expansion of the materials used to make these relatively large holding devices—relative to the coefficient of expansion of the other elements of such cards—deformations often appear on the external surfaces of finished cards that contain such electronic component holding devices. That is to say that surface deformations can result from the mere presence of such holding members in the body of the card as it experiences different temperatures and pressures during its manufacture. Such deformations are, at best, unsightly; at worst, they may prevent the card from lying completely flat in the card-receiving receptacles in certain card reading machines.
Some memory card manufacturers have dealt with this problem by reducing the size of such holding devices or by using glues to securely position their electronic components in card-forming cavities during the thermoplastic injection process. The use of glues to secure electronic components has, however, resulted in another set of problems. These problems are due to the fact that most commercially available, fast-curing glues that are used to fix such electronic components in place are often characterized by their high degree of shrinkage. Moreover, relatively large volumes of glue are needed to fix the electronic components. Use of relatively large volumes of high-shrinkage glue tend to wrinkle and otherwise deform the region of the plastic sheet or layer to which such glues are applied. This wrinkling can transmit through the thin body of the memory card and cause the outer surface of the card to take on a local wave-like character. Beyond certain tolerances, these wave-like bends are unacceptable in the memory card industry because a deformed memory card will be inoperable in certain devices.
One additional limitation, which was touched on above, in the manufacture of prior art memory cards is that they are typically produced with prior art processes that involve the injection of filled epoxy resin or high-temperature, high-pressure thermoplastic injection into molded forms. In addition to the fact that a high-pressure, high-temperature injected material may stress or damage the electronic components of the card, it also takes a relatively long time to set and cool in the mold. Epoxy resins undergo a chemical reaction following injection, which can damage the electronic components of the memory card. What is needed is a method for producing memory cards that does not require providing a “glob-top” for memory die assemblies, with a rapid cure time and rapid manufacturing cycle time, and without the use of internal holding measures that could damage the memory card electronics.